Advances in all areas of modern technology have, to a large degree, depended on the modification and development of new materials and the purification of substances used as both reagents and as materials, in the presence of which various processes are conducted. The purification of many such substances, typically liquids and gases, has required the removal of impurities which are either heterogeneous (such as particles and macromolecules) or homogeneous (such as dissolved substances). Typically, heterogeneous impurities are removed by filtration techniques and devices in which the particles are physically retained by some sort of perforate or porous medium. Other methods and purification techniques are typically chosen to remove homogeneous impurities. Many of these techniques involve the chemical modification and/or the affinity and attraction of the homogeneous impurity and the resulting removal of such material from the fluid.
In many areas of modern technology, the concentration of impurities above several parts per million (ppm) cannot be tolerated and in certain technologies, such as in the manufacture of semiconductor devices, the concentration of impurities in both the substances used as reagents as well as other materials, in the presence of which the processes are conducted, can still be detrimental even at levels at or below several parts per billion (ppb). For example, in many of the process gases employed in manufacturing semiconductor devices, impurities such as moisture, oxygen and organic compounds in even trace amounts can be adsorbed on the semiconductor wafer, causing degradation of performance, reduced manufacturing yield and adverse reliability.
In such applications, to remove homogeneous impurities from process gases, various commercial purification techniques and purifiers which involve physical adsorption and chemisorption of impurities or conversion of impurities to other forms which can be adsorbed on a solid substrate are employed. Most of these purification techniques employ a packed bed of particles or expanded materials. Examples of such materials include various resins (e.g., Nanochem.RTM. resins) and various alloys (e.g., Zr--V--Fe alloys). For purification by this technique, the gas stream is passed through these packed beds and the impurities react with the sorption material. While capable of removing impurities on the ppm level, these purification materials often do not effectively filter trace homogenous impurities, such as reactive gases, present at the ppb level. Moreover, these packed beds tend to be ineffective when there is an abrupt surge in the impurity level due to the inefficiency of the packed beds in effecting contact between impurity molecules and the resin or alloy of the bed material. In addition, because such materials themselves tend to generate heterogeneous impurities because of mechanical motion and attrition of particles of the packed bed material, they have limited service lifetimes. Furthermore, these materials are typically not reusable and often cannot be regenerated.
For the removal of particulate material, various porous ceramics have been employed. For example, U.K. Patent No. 2,201,355 to Dahlquist et al. employs a porous membrane for separating heterogeneous impurities from an aqueous medium. The porous membrane comprises an outer support matrix having through-passages and an inner layer lining the through-passages and deposited on the outer support. A polymer, metal or ceramic is used as the support matrix. The inner layer is a matrix of particles of aluminum hydroxide, partially hydrated aluminum oxide, silicon dioxide or zirconium dioxide. French Patent No. 2,251,351 describes a microporous ceramic filter that includes a microporous ceramic support electrophoretically coated with an oxide of Al, Si, Mg, Ti, Cr, Ni, Zr or Fe. U.S. Pat. No. 3,288,615 to Estes et al. describes a ceramic filter body that includes a framework of one or more tectosilicates with a mineral species (e.g., aluminates and oxides) distributed throughout and filling the framework.
While these and other filtration and purification materials permit removal of impurities with varying degrees of success from a gas stream during operation, significant potential for introduction of contamination to the system occurs during start-up periods. This is particularly true when the filtration or purification device is placed into service in the process stream. Thus, referring to FIG. 1, which illustrates in section a conventional filter device used in gas process streams, a filter is incorporated within a housing through which the gas stream flows. More particularly, the device 1 includes a fluid purification filter 3, often a reactive gas filter, disposed within the housing 5 having a fluid inlet 7 and fluid outlet 9 formed as fluid connectors at opposite ends of the housing. Such an arrangement requires that all of the gas pass through the fluid purification filter 3 before reaching the outlet 9. Prior to use, caps, typically formed from a metal material, are placed over the inlet and outlet connectors, 7 and 9, to keep dust, other particulate contaminants, and in some instances, fluids, from entering the housing. Immediately prior to use, the caps are removed so that the housing may be inserted into the system. Devices with structures of the type shown in FIG. 1, however, allow for the introduction of contaminants from a number of sources to the system.
One source of contamination originates within the housing 5 between the filter 3 and the inlet 7 from air or other fluid present in or flowing into the upstream portion of the housing through the inlet 7 when the device is installed in the process stream. In many instances, air, components of air or impurities in air, are detrimental to the process stream and the reactive gas filter is capable of removing the undesirable components or impurities present in the air which would adversely affect the process stream. Once the device is installed in the process stream and flow begins to pass from the process stream through the housing, the air initially present in or entering the housing through the inlet does not introduce further impurities to the process stream since the stream passes through the filter 3. It is, however, desirable to minimize the amount of air, contaminants, etc. entering the device since the filter 3 has a limited capacity to remove impurities. To minimize the amount of air entering the process stream at the inlet 7 when the device is installed, a valve, such as a poppet valve, could be placed within the inlet 7 and the housing pressurized with the gas employed in the process stream while the outlet of the housing is capped. However, while such an approach is effective in some instances to flush a filter device such that it may be used immediately after the device is capped, in other instances there is the potential for introduction of a second source of contamination into the process stream. Thus, depending upon the location and climatic conditions where the filter device is pressurized and the location and atmospheric pressure where the filter device is used, the gas in the downstream side of the filter housing, between the filter 3 and the outlet 9, may be displaced by atmospheric air or other fluid in the vicinity of the housing stream once the cap is removed from the outlet and the filter housing is connected in the fluid stream. Although in some instances it may be possible to flush the filter device with an inert gas or the gas employed in the process stream at the time the device is placed into service, this requires one or more additional steps.
With current technologies, in addition to sources of contamination introduced from the ambient environment of the filtration/purification assembly when the device or assembly is placed into service, contamination may be introduced by diffusion of atmospheric contamination located in the vicinity of the inlet and particularly the outlet of the housing. While the concentration of such contaminants may be very small, in particular applications even trace amounts of such contaminants may be detrimental to the processes and products involved.